Add comprehensive Hardhat test suite for ListingRegistry.sol

[dominusaxis]

Summary

Complete Hardhat test suite for ListingRegistry.sol as requested in #3.

Tests Included (30 tests across 6 categories)

Category	Tests	What is Verified
Signature Validity	3	Valid EIP-712 signatures accepted, multiple sellers, correct listing IDs
Invalid Signatures	6	Wrong signer, tampered URI, tampered timestamp, tampered seller, invalid bytes, empty sig
Event Emission	4	ListingVerified on submit, ListingRevoked on revoke/forceRevoke, correct IDs across multiple
Seller Mapping & Metadata	6	Seller address, URI, timestamp, active flag, post-revocation flag, listingCount increment
Access Control	6	Seller can revoke, non-seller rejected, double-revoke rejected, owner forceRevoke, non-owner rejected, idempotent forceRevoke
Edge Cases	6	Empty URI, zero timestamp, very long URI, max uint256 timestamp, multiple listings per seller, relay submission

Technical Details

• Uses ethers.js v6 signTypedData() for EIP-712 structured signing
• Domain matches contract exactly: name, chainId, verifyingContract (no version field)
• Hardhat config targets Solidity 0.8.20 with optimizer
• OpenZeppelin v5 compatible

Setup

npm install
npx hardhat compile
npx hardhat test

Files Added

• test/ListingRegistry.test.js — 30 comprehensive tests
• hardhat.config.js — Hardhat configuration
• package.json — Dependencies

Closes #3